spl: add overall SPL size check

spl: add overall SPL size check

This adds a size check for SPL that can dynamically check generated
SPL binaries (including devicetree) for a size limit that ensures
this image plus global data, heap and stack fit in initial SRAM.

Since some of these sizes are not available to make, a new host tool
'spl_size_limit' is added that dumps the resulting maximum size for
an SPL binary to stdout. This tool is used in toplevel Makefile to
implement the size check on SPL binaries.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>